The present invention is directed generally to spectrophotometric apparatus, and more particularly, to a system and apparatus for spectrophotometrically comparing material samples with improved efficiency.
It is advantageous in many biological, physiological and chemical analysis applications to be able to make rapid spectrophotometric comparisons of fluid samples. One such application is in tracing the source of water pollutants, since many of the commonly encountered pollutant materials such as oil exhibit unique repeatable fluorescent characteristics, readily measured by conventional spectrofluorometric techniques. The suspected material, when excited by a light of constant wavelength, responds by emitting light in another, longer wavelength. By exposing the sample to a light source of variable wavelength and plotting the intensity of the emitted light at a selected wavelength, a unique characteristic plot, or signature, can be obtained for the material.
To positively identify two samples as having the same origin, a comparison can be made between their signatures. While this can be accomplished by plotting the emission characteristics of each sample separately and then carefully comparing the resulting plots, this procedure is time-consuming and subject to error, particularly where a large number of samples must be compared and the comparison must be accomplished by non-trained personnel or under adverse conditions. Accordingly, the need has arisen for spectrofluorometric and spectrophotometric apparatus which efficiently compares two samples and provides an output indicative of the difference between the samples.
In the past, comparison of samples in spectrophotometric apparatus has been accomplished by either positioning the samples to be compared at separate positions in the apparatus and alternately redirecting the excitation light beam to each sample, or by alternately positioning the samples in the light beam. In the former method, the light beam was redirected by means of mechanical choppers or mirrors to alternately impinge on the selected one of the samples, the light emitted from the samples being directed to a common photomultiplier tube from which the output signals are electrically processed to develop the differential output signal.
Neither of these procedures has proven entirely satisfactory. In the case where the light beam is chopped, it is difficult to maintain optical uniformity between the two light paths, since variations in the positioning or characteristics of any element not common to the two paths will effect the output signal developed by the apparatus. In the case where the cuvettes are physically repositioned, the difficulty of alternately indexing the fluid samples into position without subjecting them to forces which would affect their light-emitting or light-transfer characteristics necessitated slow transfer times and short residence periods in the beam, thereby reducing the efficiency of the apparatus and the accuracy of the comparison measurement.
The present invention is directed to a system and apparatus wherein a comparison between two samples is achieved by rapid vertical positioning of samples in a single light path, thus avoiding variations while maintaining high efficiency in the measurement procedure.